Amazing Arachnids: Vinegaroons

Adapted from pages 69-74 of Amazing Arachnids:

Armed with heavy, lobsterlike claws at the front end of their bodies and shooting almost pure acetic acid out of their rear ends, vinegaroons seem to have stepped straight out of a science fiction novel. Despite their fantastic abilities, vinegaroons are perhaps the most poorly understood of the large arachnids. This may be the result of their nocturnal habits, dark nonfluorescent coloration, and the fact that they live most of their lives underground. However, the story of these enigmatic creatures is well worth the cost, albeit paid for in sleepless nights. Their story rivals and even surpasses the creations of fiction.

The common name “vinegaroon” is well chosen. The defensive spray of the vinegaroon Mastigoproctus giganteus of the southwestern United States consists primarily of acetic acid (up to 84 percent), water (10 percent), and caprylic acid (5 percent). Acetic acid is, of course, the component that gives vinegar its characteristic odor. Hydrophilic “water-loving” acetic acid in pure form simply beads up on the lipid-containing cuticle of most arthropods. But with the addition of the lipophilic “lipid-loving” caprylic acid, the spray spreads easily and penetrates into the cuticle. The caprylic acid derives its surfactant properties from a chain of 8 carbon atoms in the molecule, as compared with only 2 carbons contained in acetic acid. The acetic acid spray is produced in a pair of pygidial glands in the abdomen of the vinegaroon. Contraction of muscles in the outer layer surrounding the gland discharges the mixture as a spray from a knoblike structure called the pygidium at the base of the “tail” (called the flagellum). By bending the abdomen and rotating the knob, the vinegaroon can direct the spray with considerable accuracy, even if the target is almost directly in front of it.

As added protection, vinegaroons can defend themselves by spraying almost pure acetic acid from the pygidium, located at the base of the flagellum. By rotating the pygidium, the vinegaroon can aim the spray in almost any direction, even almost immediately in front of it. The flagellum assists the vinegaroon in accurately aiming the spray. Photo by Bruce D. Taubert.

Vinegaroons may spray repeatedly (as many as 19 times) before depleting their reserve of defensive chemicals. It takes about a day for them to recharge their reservoir. The spray has proven to serve as a deterrent to the most formidable arthropod foes such as ants. It also repels vertebrate predators such as the fierce little predaceous grasshopper mice. In contact with human skin, it may cause a burning sensation, and of course the eyes of a potential vertebrate predator such as a bird or a grasshopper mouse would be highly vulnerable to the effects of the acid.

The acetic acid is used purely as a defense weapon—not for capturing prey. A hunting vinegaroon employs tools similar to those used by scorpions for  detecting prey. A combination of sensilla (to pick up substrate vibrations) and trichobothria (to detect airborne vibrations) on the uropygid’s legs allow it to narrow down the general location of its quarry. The tiny hairs on the flagellum might also assist in this task. At the same time, the antenniform legs are extended forward, tapping the surface as the uropygid seeks out prey. Chemosensory hairs on the antenniform legs provide chemical clues as to the identity of any objects it encounters. As soon as the vinegaroon has positively identified a potential prey animal, it charges forward, grabbing with its heavy, clawlike palps. If it misses with the first try, it excitedly feels around with the antenniform legs, searching until it has once again located its quarry.

There are 4 free living instar stages before maturation, and since the young uropygid may not molt following a poor year, it may take from 5 to 7 years to reach maturity. Vinegaroons do not molt again once they are mature, and so their normal lifespan in the wild is probably in the range of 6 to 9 years. Eventually, this magnificent predator slows down due to old age, as joints stiffen and lost appendages cannot be regenerated. Perhaps even in the wild, it may actually die of old age, still a formidable predator to the end.


Amazing Arachnids
By Jillian Cowles

The American Southwest is home to an extraordinary diversity of arachnids, from spitting spiders that squirt silk over their prey to scorpions that court one another with kissing and dancing. Amazing Arachnids presents these enigmatic creatures as you have never seen them before. Featuring a wealth of color photos of more than 300 different kinds of arachnids from eleven taxonomic orders–both rare and common species—this stunningly illustrated book reveals the secret lives of arachnids in breathtaking detail, including never-before-seen images of their underground behavior.

Amazing Arachnids covers all aspects of arachnid biology, such as anatomy, sociality, mimicry, camouflage, and venoms. You will meet bolas spiders that lure their victims with fake moth pheromones, fishing spiders that woo their mates with silk-wrapped gifts, chivalrous cellar spiders, tiny mites, and massive tarantulas, as well as many others. Along the way, you will learn why arachnids are living fossils in some respects and nimble opportunists in others, and how natural selection has perfected their sensory structures, defense mechanisms, reproductive strategies, and hunting methods.

  • Covers more than 300 different kinds of arachnids, including ones new to science
  • Features more than 750 stunning color photos
  • Describes every aspect of arachnid biology, from physiology to biogeography
  • Illustrates courtship and mating, birth, maternal care, hunting, and defense
  • Includes first-ever photos of the underground lives of schizomids and vinegaroons
  • Provides the first organized guide to macroscopic mites, including photos of living mites for easy reference
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